232 DR. LOUIS VESSOT KING ON THE PROPAGATION OF SOUND IN THE FREE 
measurement of the wave-form of the sound-waves emitted by fog-signal generators 
and in studying the effect of the trumpet on the propagation losses. 
Topler and Boltzmann’s* method, based on the theory of the interferometer, 
measures the change of optical path of a beam suitably divided by a half-silvered 
mirror in such a way that one of the beams travels through air subjected to 
compression and rarefaction, while the other passes through undisturbed air. The 
two beams are brought together by a second optical mirror under conditions which 
give rise to interference fringes. The compressions and rarefactions in the sound¬ 
wave give rise to synchronous shiftings of the fringes which may either be 
photographed on a rapidly moving band of sensitive papert or examined in detail by 
a stroboscopic method. The change of density in the air at any point in the wave 
may be easily calculated in absolute units and hence the corresponding compression. 
More suitable for practical purposes is a method first employed by Raps,| which 
may be referred to as the “ oscillating or synchronous valve method.” In this case a 
valve connected to a sensitive manometer is operated by mechanical means so as to 
open and close synchronously with the period of the source of sound; if the time 
during which the valve remains open is sufficiently small compared to the period of 
the sound-wave, the pressure registered will give the compression in the sound-wave 
at some particular phase. By advancing the phase gradually it becomes possible to 
trace out the complete wave-form. By this means Raps was able to measure 
compressions as high as 0*035 atmosphere. It would seem that this method is well 
adapted to the study of the powerful sound-waves generated by modern fog-signal 
machinery. 
§ 11. Note on the Webster Phonometer. 
As a result of extensive researches carried out by Prof. A. G. Webster, of Clark 
University, a portable instrument capable of measuring the characteristics of sound¬ 
waves over a wide range of pressure amplitudes and under most severe weather 
conditions is now available for investigating the distribution of sound in the neigh¬ 
bourhood of fog-signal generators. The construction of the particular instrument 
employed by the writer § will readily be understood from fig. 2. It consists of a 
* Topler and Boltzmann, ‘Ann. d. Phys. u. Chem.,’ Ill, pp. 321-352, 1870. In an account of 
experimental work carried out in France on the measurement of sound in relation to fog-signals, Ribiere 
mentions (footnote, p. 228) that interference methods were attempted, but without success, under 
practical conditions. Preliminary experiments along these lines with the Jamin refractometer led the 
writer to the same conclusion. 
t Raps, “ Ueber Luftschwingungen,” ‘ Ann. d. Phys. u. Chem.,’ 50, pp. 193-220, 1893. 
1 Raps, “ Zur objectiven Darstellung der Schallintensitat,” ‘Ann. Phys. u. Chem.,’36, pp. 273-306, 
1888. 
§ An early form of this instrument is described by Webster as early as 1904 (see footnote, p. 216). 
Since the date of construction of the phonometer described above (1913), the instrument has been 
considerably improved by its inventor. 
